1. Technical Field
This invention relates generally to a method and apparatus for reducing particulate matter, such as smoke emissions, from the exhaust of a diesel engine, and more particularly to such a method and apparatus for reducing particulate emissions under stoichiometric or slightly rich operating conditions.
2. Background Art
Worldwide emissions regulations slated for introduction during the next five to ten years will require that diesel engines be equipped with some form of exhaust after-treatment device. Perhaps of most concern to the diesel engine industry are the proposed reductions in NOx emissions, because they are the most difficult to mitigate from the exhaust stream of fuel-lean combustion. One of the most promising technologies for NOx after-treatment in diesel engines is the xe2x80x9cLean NOx Trapxe2x80x9d (LNT). However, these devices generally require that products of fuel-rich combustion be passed through the catalyst periodically in order to regenerate the NOx trapping sites and convert the released NOx into N2 and CO2.
For example, U.S. Pat. No. 5,450,722, issued Sep. 19, 1995 to Shinichi Takeshima, et al., describes a method of NOx purification with a Lean NOx Trap (LNT) or NOx absorber. The LNT absorbs NOx during lean engine operation and desorbs and reduces the adsorbed NOx during fuel-rich operation of the engine. Similarly, U.S. Pat. No. 5,732,554, issued Mar. 31, 1998 to Shizuo Sasaki, et al., describes a device for operating a diesel engine under stoichiometric or fuel-rich conditions for the purpose of regenerating a NOx adsorber. This device involves changing the in-cylinder air motion, increasing the exhaust gas recirculation (EGR) rate, and using an in-cylinder injector to introduce the fuel in such a way as to generate a pre-mix charge prior to combustion. More recently, U.S. Pat. No. 6,082,325, issued Jul. 4, 2000 to Steffen Digeser, et al., also describes a method and system for operating a diesel engine under stoichiometric or fuel-rich conditions for the purpose of regenerating a NOx adsorber. This method and system involves changing the in-cylinder injection timing to introduce the fuel in such a manner as to generate a low-smoke rich combustion.
Other proposals for operating a diesel engine under stoichiometric or fuel-rich conditions are described in U.S. Pat. No. 5,839,275, issued Nov. 24, 1998 to Shinya Hirota, et al., and in U.S. Pat. No. 6,134,883, issued Oct. 24, 2000 to Nobuhyde Kato, et al. The Hirota, et al. patent describes a method of using multiple in-cylinder injection events to introduce the fuel in such a way as to generate stoichiometric or rich combustion. The Kato, et al. patent describes a method for determining when a NOx adsorber should be regenerated, and for assessing deterioration in the after-treatment system. This patent does not teach how an engine should be operated rich, but rather a method for determining when it should be run rich and for how long.
The above patents describe several proposals for lean NOx trap regeneration. LNT regeneration is one of the main reasons why it would be highly desirable to operate diesel engines temporarily under stoichiometric or slightly fuel-rich conditions. However, diesel combustion is very poorly suited to stoichiometric (air to fuel ratio of about 14.5:1) combustion, as it has a tendency to generate exponentially larger amounts of smoke as the A/F ratio is decreased below about 24:1 (depending upon the engine speed and engine design). The high smoking tendency of diesel engines stems from the poor air utilization of the combustion system, as the injected fuel is unable to mix with all of the air available within the combustion chamber. As a result, very fuel-rich combustion occurs locally and soot particles agglomerate easily to form visible smoke. In modern diesel engines, the smoke emissions generated at A/F ratios approaching stoichiometric are so high that it is simply not feasible to use the above proposed techniques to regenerate an LNT. In addition, U.S. Pat. No. 5,357,908 issued Oct. 25, 1994 to Shiang Sung, et al., describes a method and apparatus for separation of the light ends of a liquid fuel, as proposed as part of one embodiment of the present invention. The Sung, et al. patent proposes the use of the light components of the liquid fuel during engine starting and warm-up, and does not suggest the use of a separated fuel for operation of a compression ignition engine during stoichiometric or fuel-rich conditions.
The present invention is directed to overcoming the problems set forth above. It is highly desirable to have a method and system for temporarily operating a diesel engine under stoichiometric, or even slightly fuel-rich conditions, but with low or moderate smoke emissions. The present invention advantageously provides a means for regenerating an LNT without introducing unburned fuel into the exhaust stream of the to engine, or requiring any additional substances for operating the engine or after-treatment device.
In one aspect of the present invention, a method for operating a diesel engine under stoichiometric or slightly fuel-rich conditions includes providing a source of diesel fuel and a source of a second fuel that is readily vaporizable in an intake manifold system of the diesel engine. The second fuel is controllably introduced into the intake manifold system and mixed with air when the engine is operating under either stoichiometric or fuel-rich operating conditions, thereby providing a pre-mixed fuel mixture combining air and the readily vaporizable second fuel that is introduced into a combustion chamber of the diesel engine. The diesel fuel is then controllably introduced into the combustion chamber through an injector nozzle disposed in the combustion chamber. The diesel fuel and the readily vaporizable second fuel mixture are then combusted in the combustion chamber of the diesel engine.
In another aspect of the present invention, an apparatus for operating a diesel engine under stoichiometric or slightly rich conditions, in which the diesel engine has at least one combustion chamber, and an air intake system and an exhaust gas system in communication with the combustion chamber, includes a first injection nozzle disposed in the combustion chamber of the engine and a second injection nozzle disposed in the air intake system of the engine. The first injection nozzle is in communication with a source of diesel fuel, and the second injection nozzle is in fluid communication with a second fuel that is readily vaporizable in the air intake system. At least one sensor is provided for sensing the relative ratio of the air/fuel mixture introduced into the combustion chamber. A controller, in communication with the sensor and with the first and second fuel injection nozzles, controls the injection of the diesel fuel through the first injection nozzle and the injection of the readily vaporizable second fuel through the second injection nozzle in response to the controller determining that the fuel/air mixture in the combustion chamber is either a stoichiometric or a fuel-rich mixture.